1. Field of the invention
The invention relates to a semiconductor component containing at least one pn junction which extends inside a semiconductor substrate and which is formed by a laterally bounded, highly doped zone extending inwards from a main surface of the semiconductor substrate and by a lightly doped zone surrounding the highly doped zone and emerges at the main surface of the semiconductor substrate at an edge of the highly doped zone, the edge of the highly doped zone being formed by a guard zone whose doping density gradually decreases in a direction parallel to the main surface from the highly doped zone towards the pn junction.
2. Discussion of background
In the regions where the reverse-blocking pn junction emerges at the surface of the semiconductor substrate, semiconductor components designed for high reverse voltages require special measures to ensure the blocking capacity of the semiconductor component. The reason for this is that, without such measures, the surface breakdown already takes place long before reaching a reverse voltage which is limited by the volume breakdown and thus reduces the dielectric strength of the semiconductor component in an unacceptable manner.
As is evident from the publication entitled "Methods of avoiding edge effects on semiconductor diodes", by P. A. Tove, J. Phys. D: Appl. Phys, 15 (1982), a large number of very varied measures of this type are now known which all have the object of reducing as greatly as possible the maximum field strengths occurring in the region where the reverse-blocking pn junction emerges at the surface so that the volume breakdown takes place, if possible, before the surface breakdown.
The known measures may be divided roughly into two categories, the first comprise measures which provide an external structure of the surface (grinding edge profiles, edging trenches) and the second comprise those measures which are based solely on a suitable internal doping structure with the surface remaining intact.
For semiconductor components having fine structures or having integrated MOS structures which permit the use of the dividing technique in their production, those measures are primarily of importance which leave the surface of the semiconductor substrate intact. These include, for example, guard rings and VLD structures (VLD=Variation of Lateral Doping). Where as guard rings fulfil their intended function only if they are very precisely dimensioned and manufactured, VLD structures are a rugged alternative.
The use of a VLD structure to increase the dielectric strength is known from "Variation of lateral doping--a new concept to avoid high voltage breakdown of planar junctions", by R. Stengl and U. Gosele, IEEE 1985. At an edge of a highly doped zone, the doping density is reduced linearly in a direction parallel to a main surface of the semiconductor substrate from the doping density of the highly doped zone to that of a slightly doped zone. Such a so-called lateral gradient of the doping density of the VLD structure is produced in a manner such that dopant is implanted through a mask having slots and gaps with precisely adjusted dimensions and a preliminary predeposition produced in this manner is diffused in.
In this manner, semiconductor components can be realized whose reverse dielectric strength is about 90 percent of the volume breakdown. Even if such values are good for the reverse dielectric strength compared with other arrangements, they do not represent the desirable optimum. In addition, the production of such a VLD structure presents a problem since, in the known process of P. A. Tove, even small irregularities in the mask result in undesirable undulations of the pn junction which lower the reverse dielectric strength of the semiconductor component.